This invention relates generally to apparatus and methods for providing electrical power to electrical circuits located down in a well. The invention relates in a particular aspect to a power supply for, and related method of providing electricity to, one or more electrical circuits disposed in a well having a downhole temperature within the range between about 300.degree. F. and about 400.degree. F. and having a minimum diameter of about 6 inches through which the power supply is to be lowered with the electrical circuit(s).
In drilling and completing oil and gas wells or geothermal wells, for example, tools which have electrical circuits typically need to be lowered into the borehole such as for taking measurements of pressure and temperature, for example. Electricity must be provided to these circuits for them to operate.
One manner of providing electricity downhole in a well includes lowering the tool on a wireline and conducting energizing electricity through one or more conductors in the wireline from the surface to the tool positioned downhole. This technique is not always desirable because it is relatively complex in that it requires the wireline to be passed through the wellhead closure equipment at the mouth of the well. This can create safety problems. Furthermore, at least in deep wells, there can be significant energy loss caused by the resistance or impedance of a long wireline conductor.
Another way to provide electricity to downhole electrical circuits utilizes batteries housed with the electrical circuits in the downhole assembly For example, lithium-thionyl-chloride batteries have been used with downhole tools. A shortcoming of batteries, however, is that they cannot provide moderate (and higher) amounts of electrical energy (e.g., 30 kilowatt-hours) at the elevated temperatures encountered in petroleum and geothermal wells.
Because downhole tools having electrical circuits are extensively used in at least the oil and gas industry, there is the need for an improved apparatus and method for providing electricity to such circuits when they are in a well. Such apparatus and method should be localized with the electrical circuits to overcome the noted shortcomings of the wireline technique, and they should provide desired levels of electrical energy at the elevated downhole temperatures to overcome the noted shortcoming of conventional batteries.